STRUCTURE AND BINDING-SITE OF THE PRIMARY ELECTRON-ACCEPTOR IN THE REACTION-CENTER OF CHLOROBIUM

In isolated, chlorosome-free reaction centers from Chlorobium limicola f thiosulphatophilum, a chlorin pigment exhibits a Q(y) absorption band at 672 nm (Feiler, U., Nitschke, W., and Michel, H. (1992) Biochemistry 31, 2608-2614). To characterize the chemical nature of this chlorin pigment and its interactions within the reaction-center protein, selective enhancement of its Raman scattering was achieved by resonant excitation within its Soret band. This is the first time that structural studies of this pigment were performed on the native reaction-center protein. The obtained resonance Raman spectra were consistent with a single population of a chlorophyll a(-like) pigment, possessing a vinyl group on ring I, but not with bacteriochlorophyll c or bacteriophaeophytin c. The stretching frequencies of the C-9-keto carbonyl of this pigment indicates that it is H-bonded to the reaction-center protein. The strength of this H-bond is very close to those of the keto carbonyls of the primary electron accepters in purple bacterial reaction centers and D1/D2 particles. Since in membranes of Chlorobiaceae a transient bleaching at 670 nm is due to the primary acceptor in the reaction center (Nuijs, A. M., Vasmel, H., Joppe, H. L. P., Duysens, L. N. M., and Amesz, J. (1985a) Biochim. Biophys. Acta 907, 24-34), we thus conclude that the primary acceptor in Chlorobium reaction centers is the characterized chlorophyll a(-like) pigment. This pigment might be identical with the so-called BChl-663 molecule which was observed in from Chlorobiaceae (van de Meent, E. J., Kobayashi, M., Erkelens, C., van Veelen, P., Otte, S., Inoue, K., Watanabe, T., and Amesz, J. (1992) Biochim. Biophys. Acta 1102, 371-378.).